I. Field of the Invention
This invention relates to a radiation dosage indicator, and in particular, to a dosimeter having a radiation sensitive zone capable of changing opacity in response to exposure to radiation to change the visibility of indicia on said indicator, and to the method of manufacturing such indicator.
II. Description of the Prior Art
The use of dosimeters to determine the absorbed dose of ionizing radiation received by the person or the substrate to which it is attached is well known. There are many types of dosimeters, the most common being the air-capacitor dosimeter, the film badge and the thermoluminescent dosimeter.
The air-capacitor dosimeter includes an inside chamber which serves as an ion chamber, and a central collecting electrode. An electric charge is placed on the central electrode. Such dosimeters, when charged, are essentially air-capacitors, and the amount of discharge during use is proportional to the absorbed dose of X-ray or gamma ray radiation received. Inherent in the use of such dosimeters is the need of a power source for applying a voltage between the electrode and the chamber wall.
A film badge dosimeter usually is loaded with one or more film packets. The simplest type of film badge consists of a small paper envelope containing a dental film, one-half of which is surrounded by a thin lead foil. The badge must contain one or more filters so that a comparison can be made of the relative blackening of the developed film from behind the various filters. This comparison reveals the extent of exposure to various types of radiation. Inherent in the use of such badge is the need of a developing process to develop the exposed film.
A thermoluminescent dosimeter is one that functions on the principle of thermoluminescence; that is, the property of certain substances that release light upon heating after they have been exposed to ionizing radiation. Inherent in this type of dosimeter is the need for measuring either the peak intensity of the light emitted or the integrated quantity of light.
While such dosimeters perform satisfactorily, they require outside equipment or processes in order to function and/or be readable. That is, one cannot detect merely by visually observing the exposed material, the level of absorbed radiation.
It also is known to make dosimeters having a substance which changes color when exposed to ionizing radiation. For example, in U.S. Pat. No. 4,001,587 to Georgy Mitrofanovich Panchenkov et al dated Jan. 4, 1977, dosimeters are disclosed which incorporate various dyes, some acid-sensitive and some not acid-sensitive, which change their color on exposure to radiation. Generally, one requires the use of outside equipment, such as a spectrophotometer, to determine the level of radiation.
In U.S. Pat. No. 4,125,534 to Kwok C. Yee dated Nov. 14, 1978, there is disclosed that diacetylenes are known to react upon exposure to high energy irradiation, such as gamma rays, and undergo a color change indicating polymerization. However, here again, the use of a spectrophotometer is generally required for determining dosage.
In U.S. Pat. No. 4,389,217 to Ray H. Baughman et al dated June 21, 1983, the indicator comprises a substrate having deposited thereon at least two indicating units capable of color change upon thermal annealing or exposure to actinic radiation. One indicating unit is unreacted and represents zero time of exposure and the other of such units is pre-reacted to a known extent whereby the time required by each unit to form the end-point color at a given average radiation dosage rate is predetermined.
In U.S. Pat. No. 4,494,003 to Adolph G. Hager et al dated Jan. 15, 1985, there is disclosed the use of glass doped with iron or manganese which is exposed to gamma radiation. The amount of gamma radiation detected by the glass is measured by an instrument providing a fixed, calibrated source of light. That is, one can measure the attenuation of light transmitted through the gamma irradiated sample of glass as a function of gamma dosage, or one can measure the amount of gamma radiation as a function of the change in the index of refraction of light transmitted through the irradiated sample of glass. The amount of radiation also can be measured by comparison with a color chart.
In U.S. Pat. No. 4,788,126 to Lyudmila Feldman et al dated Nov. 29, 1988, there is disclosed as prior knowledge that some type of dosimeters require chemical processing, either in a laboratory or at the place of radiation exposure. This patent provides for microcapsules which are subjected to a rupturing force such that the image-forming agent is released. The color shade or density of such agent is compared with a reference image to determine the amount of exposure.
In U.S. Pat. No. 4,791,155 to David P. Gregory et al dated Dec. 13, 1988, there is disclosed known radiation indicators utilizing acid-sensitive dyes to monitor the radiation.
Still further, in U.S. Pat. No. 3,768,976 to Kwoh H. Hu dated Oct. 30, 1973, there is disclosed a high-temperature integrating indicator in which a redox dye is colored in its reduced state, but which turns clear in the oxidized state due to the diffusion of oxygen into the device over a predetermined time interval. The disappearance of the color reveals a warning message as to the condition of a food product kept under storage to which the indicator is attached.
The present invention improves on the heretofore known dosimeters by providing an indicator which undergoes a color change upon being irradiated to change the visibility of indicia included as part of the indicator. This serves to provide a visual indication, without additional processing or instrumentation, as to whether the substrate to which the indicator is attached has been exposed to a radiation dosage exceeding a predetermined threshold.